Atomic number of neodymium

SIXTY

This rare-earth element’s place on the periodic table is a round number between 55 and 65.

Neodymium is a rare-earth element with the atomic number 60, so the answer is simply the word form of that number. The clue is straightforward—no wordplay or tricks—just a direct reference to its position on the periodic table. "Sixty" is the spelled-out version of 60, matching the crossword’s answer format.